Compact stacked fin heat exchanger

ABSTRACT

A method and system is disclosed in which an air to air heat exchanger keeps two air streams separate through the use of metal fins. Edges of alternating fins are folded so that the end is in the proximity of the adjacent fin and the remaining gap is sealed. The fin stack is held together by interlocking features or through the use of a tube, in which case the fin stacks may be stamped and stacked on alignment stakes during production using the same holes as the tubes. The heat exchanger may have extended use in removing moisture and heat from a hot humid air stream. The heat exchanger has utility in heat recovery ventilation systems, clothes dryer heat recovery or other air to air heat exchanger processes where the pressure difference between the airstreams is less than approximately 5000 Pa.

PRIORITY STATEMENT UNDER 35 U.S.C. §119 & 37 C.F.R. §1.78

This non-provisional application claims priority based upon prior U.S. Provisional Patent Application Ser. No. 62/130,063 filed Mar. 9, 2015 in the name of Jeremy Rice entitled “Compact Stacked Fin Heat Exchanger,” the disclosure of which is incorporated herein in its entirety by reference as if fully set forth herein.

BACKGROUND OF THE INVENTION

Air to air heat exchangers are used in many heat transfer and heat recovery applications. In applications where heat is desired to be transferred between two separated airstreams without the transfer of moisture, metal foil, often aluminum, is used as the boundary to prevent mixing between the two airstreams. As depicted in FIG. 1, at the edge of the fins 103, it is necessary to block airflow 100 from entering the gap between one pair of fins 103, while allowing the same stream of airflow 101 to enter the neighboring gap. This alternating pattern of allowing airflow to enter only pre-determined channels keeps a first airflow stream 101 and second airflow stream 102 separate, and allows heat to be transferred via the metal foil 103.

One method that is often used to prevent air 100 from entering a predetermined gap is rolling 104 the edges of adjacent fins 103, 106 to create a mechanical seal preventing airflow from one airstream 100 to enter that particular channel. While this method is used quite frequently, the formation of the rolled edge 104 is a time consuming process, since the edge can't be formed with a simple up/down progressive stamping machine, which can add to the product cost. Additionally, the fin pitch 105 has to be relatively large to allow for the roll 104 to be formed. The lower manufacturing limits on this pitch is around 2.0 mm, but more typically heat exchangers are designed for fin pitches of 4 mm to 8 mm fin pitch. The result of the higher fin pitch and time it takes to create the rolled edge 104 is the heat exchanger's volumes are relatively large for the airflow and heat exchanged. The heat exchangers tend to be long in the direction of the airflow paths 101, 102 which leads to higher pressure losses.

SUMMARY OF THE INVENTION

The present invention enables tight fin pitches, down to approximately 1.0 mm, for air to air heat exchangers, with air streams separated by metal foil. These fin pitches can enable twice the fin density and twice the heat transfer coefficient, leading to a quadrupling of the heat transfer that may be exchanged in the same volume. The formation of the fin stack and the sealing of the alternating channels, is separated, which allows for high manufacturing process efficiency.

In the present invention, there are two major process steps, the fin stack formation and the sealing of the edges with a spray coating process. In the fin stack formation step, the stack is mechanically held together by interlocking fins, in which each fin interlocks with the adjacent fin. Alternatively, the fin stack can be held together by several rods or tubes that pass through the interior of the fins with an interference fit. The fins can either press fit onto the rods, or, alternatively, the tubes can be expanded. In either case, the interference fit maintains the fin spacing. After the mechanical forming process, the edge of every other fin is formed into a 90 degree edge that is intended to touch the neighboring fin, and thus block air from entering the channels adjacent to this 90 degree formation. The coating process helps seal the remaining gaps on the edge that remain as a result of tolerances on the creation of the formed edge. Additionally, the sealing helps bond the formation and neighboring fin together, thus strengthening the fins.

The foregoing has outlined rather broadly certain aspects of the present invention in order that the detailed description of the invention that follows may better be understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic drawing of a rolled edge design in accordance with prior art;

FIG. 2 is a top perspective view of the heat exchanger and airstreams in one embodiment of the present invention;

FIG. 3 is a close-up view of the top front corer of the heat exchanger and airstreams of the foregoing embodiment;

FIG. 4 is a schematic drawing depicting one embodiment of the edge sealing of stacked fins by a coating process;

FIG. 5 is a schematic drawing depicting a fin stack before the coating process;

FIG. 6 is a schematic drawing depicting the fin stack of FIG. 5 after the coating process is complete;

FIG. 7 is a schematic drawing depicting a fin stack held in place by interlocking fins;

FIG. 8 is a schematic drawing depicting a fin stack held in place by a tube;

FIG. 9 is a schematic drawing depicting a lap joint sealing of the stacked fins by a coating process;

FIG. 10 is a schematic drawing depicting a fin stacking method onto a rotating table with a single fin type;

FIG. 11 is a schematic drawing depicting a fin; and

FIG. 12 is a schematic drawing depicting a fin stacking method utilizing two stamping presses.

DETAILED DESCRIPTION

The present invention is directed to a compact stacked fin heat exchanger. The configuration and use of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of contexts other than a stacked fin heat exchanger. Accordingly, the specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

A representation of one embodiment of the present invention is presented in FIG. 2. Two separate airstreams enter the heat exchanger 107 and transfer heat without mixing. A first airstream 101 has a relatively high temperature when entering the heat exchanger 107, and releases heat to a second airstream 102 that has a relatively low temperature when entering the heat exchanger 107. As the first airstream 101 releases heat, its temperature reduces, while the second airstream 102 increases in temperature as it picks up heat. The first airstream 101 and the second airstream 102 are separated by a series of fins 108, as is represented in a close-up depiction of one embodiment of the invention in FIG. 3. The first airstream 101 is blocked by a series of alternating folded edges 109 on every other fin 108, preventing the first airstream 101 from mixing with the second airstream 102. The second airstream 102 is blocked by a second series of alternating folded edges 110 preventing the second airstream 102 from mixing with the first airstream 101.

A schematic of the folded edged 109 is represented in FIG. 4. The edge 109 is part of a fin or plate 114 that keeps the first airstream 101 and second airstream 102 separate. The edge 109 is created with a 90 degree bend from the fin 114, and nearly touches the neighboring fin 113. Since the fin 114 is made from a mechanical process, the edge 109 and the neighboring fin 113 do not come into perfect contact, therefore a small gap 112 is created. Since the first airstream 101 and the second airstream 102 are desired to not mix, the gap 112 must be sealed. A secondary coating process, such as spray paint or powder coating, may be applied to the surface. This coating process creates a film 111 that can penetrate and seal the gap 112, thus preventing the first airstream 101 and the second airstream 102 from mixing.

It may be required to apply the coating in multiple passes to ensure the coating penetrates the gaps. If a powder coating process is used, a thicker coating may be used than conventional liquid coatings, without running. The powder coating process will need to be cured so that the particles (powder) can melt and bond to the base surface. Since the coating bonds to the base surface (fin), it has a beneficial effect of strengthening the fin's edge. Since the metal fin is thin (0.1 mm to 0.5 mm thick), the stiffness of the edge increases significantly as a result of bonding two flat fins with a 90 degree connection. This stiffening is similar to an I-beam element used to strengthen structures.

A view of a fin stack with edges 109 formed by a progressive stamping process is represented in FIG. 5. Since the fins may be created by a thin piece of metal foil ranging from 0.1 mm to 0.5 mm thick, the strength of the fin is limited. Due to the fin's limited strength, the fin is subject to deformation, thus creating a gap 112 of varying size. In an effort to minimize this gap 112, a spacer feature 115 can be added to the gaps in which an airstream 101 is allowed to flow through. This spacer 115 limits the span in which the edge 109 is not reinforced, and helps keep the gap 112 within a tolerance that may be sealed by the coating process. A view of the fin stack after the coating process is presented in FIG. 6. In this image, there are no more visible gaps 112 indicating that a seal was formed, and mixing can be prevented between the two airstreams.

The benefits of having a simple 90 degree edge 109 with a separate coating 111 vs a rolled edge 104 is that the metal formation process is readily attainable by a progressive stamping process, whereby, each fin 114 may be created independently from the previous fin 114 and stacked. Additionally, the fins 114 may have a tighter fin pitch, down to 1.0 mm, versus a rolled edge, which has a lower bound fin pitch of approximately 2.0 mm. The tighter fin pitch enables twice the fin area per unit length as well as twice the heat transfer coefficient from airstream 101 to fin 114, since the distance from the bulk airstream temperature to the fin 114 is half. The combined effect of the increased fin area and heat transfer coefficient is that the same total amount of heat may be transferred with approximately 25% of the heat exchanger length.

The final issue that must be addressed is maintaining the fin stack's form prior to the coating process. This issue may be addressed in multiple ways. In a first method, represented in FIG. 7, the fin stack may be held together by interlocking features 116. These features keep the fins 114 from being pulled apart as well as being pressed together. The interlocking feature can be used in lieu of or in conjunction with the spacers 115. The interlocking features must be present on opposing sides of the fin, to ensure the fin stack is secure in all three dimensions.

In a second method, as represented in FIG. 8, the fin stack may be held in place by a tube 120 or tubes. The tube and fins can be forced together with an interference fit, which may be created by two techniques. The first technique, the cut-out in the fin, which the tube penetrates, may have an internal diameter that is smaller than the outer diameter of the tube. The fin may be pressed onto the tube, creating a friction or interference fit between the tube and the fins. The process may be repeated to create the fin stack. In a second technique, the diameter of the cut-out in the fin may be larger than the outer diameter of the tube. The fin stack may first be loosely stacked onto the tube (or tubes). Once the stack has the desired amount of fins, the stack may be compressed with a controlled amount of force to get a close fit on the fin's edges 109, but not too much force to deform the fins. Finally, the tube may be expanded until the outer diameter of the tube is larger than the cut-out diameter in the fin, thus creating an interference fit.

The shape of the fins, and thus the heat exchanger, is highly customizable by the methods set forth herein. Rectangular, hexagonal, circular and many other shapes may be used. Additionally, the airstreams may be designed to flow in a cross-flow or counter-flow pattern, depending on the application and constraints. Additionally, the fin pitch on each airstream may be different. This feature may be useful in situations where one air stream has a high humidity level or even a higher volumetric flow with respect to a second air stream, in which the heat transfer requirements are not balanced. In the case with an air stream with a high humidity level, condensation is likely to occur when that airstream is cooled. Since the sealing process is performed at relatively low temperatures, fins with a hydrophilic coating may be used, so that the condensed water does not form droplets on the fins. Additionally, this feature gives flexibility in designing the heat exchanger, where one air stream may have a relatively large cross-section for entering air, the second may have a smaller cross-section. Each application will require its own optimization.

The fins described so far are generally flat and thin. The general construction of the heat exchanger may be kept the same, with features added to the fins to enhance the heat transfer, as well as increase the structural integrity of the fins. Some of these features include ribs or waves, than can be implemented in a variety of manners. Embossing features may be used that are tall enough so that they touch the neighboring fin in the middle of the channel. These features can extend the allowable pressure difference between the air streams. Since the foil is relatively thin, it is expected that the useful pressure difference between airstreams is limited by approximately 5000 Pa.

Another embodiment is presented in FIG. 9, which includes a first air stream 101 and second air stream 102, which are separated by plates 117 and 118. At the entrance, a lap joint 119 prevents the first airstream 101 from mixing with the second airstream 102. A coating 111 may be applied to the face of the heat exchanger, thus creating a seal in the lap joint 119 between a first plate 117 folded under and a second plate 118 folder over. In some embodiments, the fold on the second/outer plate 118 is shorter than the fold on the first/inner plate 117, thereby exposing a portion 121 of the outer surface of the inner fin 117 for the coating to land.

In the method where the plates of a heat exchanger are integrally held together by a tube or tubes 120, the plates may be stamped on a stamping press 201 and stacked onto the alignment stakes 121 which are held in place on a table 203, as represented in FIG. 10. The alignment stakes go into the same holes 210 in the plate as the tubes 120 but can have a smaller diameter than the tubes, or a pointed tip, allowing for easier placement of the fins. The sheet metal flows 202 from a roll, to the press 201 and then onto the table 203. The tubes 120 can replace the stakes 121 during another process and then can be expanded to form an interference fit with the plates. In this process, a single plate type is stacked onto the stakes, and the table 203 is rotated in 90 degree alternating clockwise and counterclockwise directions, in the time between the placement of each individual plate.

One embodiment of a plate utilized in this process is represented in FIG. 11. The main surface 209 of the plate is generally square, however, a first direction 204 is shorter than a second direction 205 by two times the plate thickness plus stamping and stacking tolerances. The first set of edges 206 on the opposing ends of the first direction 204 are folded upward, while the second set of edges 207 on the opposing ends of the third direction 205 are folded downward. The second set of edges 207 are shorter than the first set of edges 206. The second set of edges 207 form the outer surface of the lap joint 119, while the first set of edges 206 form the inner surface of the lap joint 119. The plate has holes 210 that must align with the stakes 121 during the stacking process. The location of the holes 210 must align to the same location, on a fixed reference plane, when the plate is rotated by 90 degrees. The plates are often a metal foil, therefore, embossings 216 may be used to maintain a desired spacing, add strength and increase the heat transfer effect on the fins.

Alternately, hexagonally shaped plates may be used for air streams that are desired to flow counter to each other rather than cross each other. A similar fin stacking and table rotating approach may be used for a hexagonal fin, however the table must be rotated 180 degrees between the stacking of the plates versus 90 degrees.

There may be situations where a single fin type cannot be used to obtain the desired geometry, such as rectangular cross-flow heat exchangers and varying fin pitch heat exchangers. In these situations, two presses may be used to create a single heat exchanger, as presented in FIG. 12. The first fin type can flow 213 from a metal coil through a first press 211 onto a stacking table 215. A second fin type can flow 214 from a second metal coil through a second press 212 onto the same stacking table 215. The presses have to be timed to stack fins in an alternating sequence from the first press 211 followed by the second press 212.

In an effort to speed production of these heat exchangers, the stacking table 215 may allow for stacking of two heat exchangers in parallel, and rotate in 180 degree increments, to allow for both presses to run continuously.

While the present system and method has been disclosed according to the preferred embodiment of the invention, those of ordinary skill in the art will understand that other embodiments have also been enabled. Even though the foregoing discussion has focused on particular embodiments, it is understood that other configurations are contemplated. In particular, even though the expressions “in one embodiment” or “in another embodiment” are used herein, these phrases are meant to generally reference embodiment possibilities and are not intended to limit the invention to those particular embodiment configurations. These terms may reference the same or different embodiments, and unless indicated otherwise, are combinable into aggregate embodiments. The terms “a”, “an” and “the” mean “one or more” unless expressly specified otherwise. The term “connected” means “communicatively connected” unless otherwise defined.

When a single embodiment is described herein, it will be readily apparent that more than one embodiment may be used in place of a single embodiment. Similarly, where more than one embodiment is described herein, it will be readily apparent that a single embodiment may be substituted for that one device.

In light of the wide variety of methods for constructing stacked fin heat exchangers known in the art, the detailed embodiments are intended to be illustrative only and should not be taken as limiting the scope of the invention. Rather, what is claimed as the invention is all such modifications as may come within the spirit and scope of the following claims and equivalents thereto.

None of the description in this specification should be read as implying that any particular element, step or function is an essential element which must be included in the claim scope. The scope of the patented subject matter is defined only by the allowed claims and their equivalents. Unless explicitly recited, other aspects of the present invention as described in this specification do not limit the scope of the claims. 

What is claimed is:
 1. An air-to-air heat exchanger consisting of: a plurality of adjacent fins with a first end aligned on a first axis and a second end aligned along a second axis, wherein the second ends of alternating fins are folded so that the second end of each alternating fin is proximate to the adjacent fin, thereby creating a first set of channels between the fins; and wherein first ends of alternating fins are folded so that the first end of each alternating fin is proximate to the adjacent fin, thereby creating a second set of channels between the fins; a first air stream passing through the first set of channels; and a second airstream passing through the second set of channels.
 2. The air-to-air heat exchanger of claim 1, wherein the first axis is horizontal and the second axis is vertical.
 3. The air-to-air heat exchanger of claim 1, wherein the first axis is horizontal and the second axis is not vertical.
 4. The air-to-air heat exchanger of claim 1, wherein the gap formed by placing each alternating fin proximate to the adjacent fin is sealed with a sealant.
 5. The air-to-air heat exchanger of claim 1, wherein the second ends of alternating fins and adjacent fins are folded to form a lap joint.
 6. The air-to-air heat exchanger of claim 1, wherein alternating fins and adjacent fins are held together by one or more tubes passing through a hole located on the interior surfaces of each fin.
 7. The air-to-air heat exchanger of claim 1, wherein alternating fins and adjacent fins are held together by a protrusion on one fin that interlocks with a protrusion on an adjoining fin.
 8. The air-to-air heat exchanger of claim 1, wherein the gap formed by placing each alternating fin proximate to the adjacent fin is sealed with a powder coating.
 9. The air-to-air heat exchanger of claim 1, wherein all fins are formed and folded on a single stamping press.
 10. The air-to-air heat exchanger of claim 1, wherein all fins are formed and folded on a single stamping press and alternating fins are rotated 90 degrees before assembling the heat exchanger.
 11. The air-to-air heat exchanger of claim 1, wherein all fins are formed and folded on a single stamping press and alternating fins are rotated 90 degrees and stacked onto stakes secured to a table that can rotate back and forth 90 degrees in an alternating sequence as the alternating fins are placed onto the stakes during assembling the heat exchanger.
 12. The air-to-air heat exchanger of claim 1, wherein a first set of fins are formed on a first stamping press and a second set of fins are formed on a second stamping press and the first set of fins and the second set of fins are alternatingly stacked onto stakes secured to a table during assembling the heat exchanger. 